This invention relates generally to electronic image processing and more particularly to a method and system for estimating sensor dark current drift, sensor non-uniformities, and illumination non-uniformities.
During the scanning of photographic images from film, various factors may affect the quality of the resulting digital image. For example, systems used to derive digital images from film may suffer from both sensor and illumination non-uniformities, each of which may adversely affect the signal integrity of the images. Image quality may also depend, in part, on the characteristics of the film. Where digital image data is obtained from developing film, the characteristics of the developing chemical applied to the film may also affect image quality.
For example, processing images from film typically includes capturing digital data from the film with a sensor as the film is illuminated with a light source. Because the illumination levels captured by the sensor represent the image data, any sensor or illumination non-uniformities introduce undesirable errors into the data measurements. Unfortunately, sensor and illumination characteristics typically vary individually and can drift over time due to factors such as heating and/or transient responses in the devices when they are activated and deactivated. Where the film is scanned while being developed, variances in film and chemical developer characteristics often arise due to changes that take place during the development process. Signal levels captured by the sensors may also vary due to factors such as aging of the sensors or light sources.
Some systems include a calibration process using an external reference, such as a piece of film, in an attempt to correct some of these non-uniformities. The external reference is typically chosen to approximate the characteristics of the film before it is developed, and may be imaged periodically to recalibrate and correct for any non-uniformities throughout the development process. However, such external references may not accurately reflect the characteristics of film and/or developer chemicals while or after the film develops. They may also not reflect sensor or illumination characteristics that change during the development process.
From the foregoing, it may be appreciated that a need has arisen for providing a method and system for estimating sensor dark current drift, and for estimating sensor and illumination non-uniformities. The present invention substantially reduces or eliminates disadvantages and problems of existing systems.
One aspect of the present invention is a system for estimating sensor and illumination non-uniformities. The system comprises a first light source and a first sensor operable to capture light reflected from a first side of film illuminated by the light source while the film has a developer chemical applied thereto and processing circuitry coupled to the first sensor. The processing circuitry is operable to capture a first plurality of readings from the sensor responsive to light reflected from an unexposed region of film to determine a first set of non-uniformity data and adjust image data obtained from the film in response to the first set of non-uniformity data. In a further embodiment, the processing circuitry is further operable to dim the first light source for at least a portion of the time that the sensor is being used to sense the unexposed region of the film. The processing circuitry may also capture a second plurality of readings from the sensor while the first light source is dimmed to determine a second set of non-uniformity data and adjust image data obtained from the film in response to the second set of non-uniformity data.
Another aspect of the present invention is a system for estimating sensor dark current drift. The system comprises a first light source and a first sensor operable to capture light reflected from a first side of film illuminated by the light source while the film has a developer chemical applied thereto and processing circuitry coupled to the first sensor. The processing circuitry is operable to dim the light source for at least a portion of the time that the sensor is being used to sense a region of the film and to capture a first plurality of readings from the first sensor while the first light source is dimmed to determine a first set of non-uniformity data. The processing circuitry is also operable to adjust image data obtained from the film in response to the first set of non-uniformity data. The invention provides several important advantages. Various embodiments of the invention may have none, some, or all of these advantages. The invention may effectively utilize one or more unexposed regions of film while the film is developing. These regions may provide a window of opportunity to measure data that may be used to make corrections and/or adjustments to initial estimates of sensor, illumination, film, and/or developing chemical non-uniformities. The invention also allows correction and/or adjustment for non-uniformities without utilization of an external absolute reference. The invention may also utilize additional sensor views to capture data through various incident angles of light. These advantages may improve the accuracy of image data.
The invention may also use one or more unexposed regions to match the white level to one or more sensors each associated with a film development time. White level adjustment allows better use of the dynamic range of the sensor, resulting in a more accurate digital representation of the image.
The invention may also substantially reduce errors from variations in dark current levels that vary faster than the total processing time of a roll of film. For example, the invention may sample dark current levels during one or more unexposed regions while the film is being developed. Alternatively or in addition, the invention may also accommodate sampling of dark current levels within a sampling time for a latent image. The invention may also compensate for drift or other variations in sensor or illumination source characteristics between image capture sampling times. The invention may also adjust the captured image data to compensate for such variations. For example, sensor and illumination non-uniformities in the image data may be normalized and various gains for the image data may be adjusted using the measured data.
In addition, transient responses of devices can be reduced, as can the time for devices to reach a proper operating point. The invention may also generate an alert that illumination and sensor devices should be replaced. The invention may also prevent saturation of sensors in varying film, developer, and illumination conditions.
Other technical advantages may be readily ascertainable by those skilled in the art from the following figures, description, and claims.